Such a connection device is described both in WO 2004/029497 A1 and in WO 2005/028939 A1. In this known embodiment, the insert part is in the form of a sleeve and is inserted into an enlarged portion of the receiving opening of the base part so as to be circumferentially sealing at least with respect to the ingress of dirt or such contaminants. The insert part in the inserted state lies completely within the base part so as to terminate flush on the mouth side. Furthermore, the insert part comprises a first, relatively harder dimensionally stable plastic material, a dirt seal being integrally formed cohesively from a second, relatively softer and elastic plastic material in the free, mouth-side end region of the spring arms. The insert part is therefore in the form of an integral multicomponent molding made from plastic. The spring arms are formed by longitudinal slots of the sleeve-shaped insert part, the longitudinal slots likewise being filled with the injected material of the dirt seal. In the known connection device, it has been shown in practical use that it is sometimes very difficult to detach the conduit by removing the insert part from the base part.